1. Field of the Invention
The present invention relates to a microscope objective that uses only mirror optics to obtain high magnification and to a reflecting microscope objective that has variable magnification.
2. Background of the Invention
The most common reflecting microscope objective is a Schwarzschild Cassegrainian reflecting objective. This design of objective offers good image quality over a wide range of wavelengths of radiant energy. The reflectivity of mirror surfaces does not change significantly with wavelength. In contrast, a lens comprises refractive material that focuses different wavelengths of radiant energy at different foci, transmits different wavelengths of radiant energy with different efficiency and may not transmit certain wavelengths at all. The ability to image radiant energy at different wavelengths is important for contemporary microscopy because a sample is often examined with radiant energy at wavelengths ranging from the vacuum ultra violet to the far infrared.
The conventional Schwarzschild Cassegrainian microscope objective, however, is useful only at relatively low magnification because the working distance between the sample and the secondary mirror becomes too small at high magnification and high numerical aperture. High magnification requires the use of small mirrors. Decreasing the size of the mirrors, however, creates problems in manufacturing due to the extremity small size of the secondary mirror. Known designs for a reflecting microscope objective cannot easily obtain a practical magnification of 100 times.
High magnification can be obtained using remote field optics to extend the focus of a reflecting objective. Remote field optics, however, are awkward and tend to reduce image quality. No known reflecting microscope objective combines good image quality with high magnification in a practical optical configuration.
It has long been known that radiant energy can form a variety of different optical paths in a Cassegrainian mirror optical system. For example, U.S. Pat. No. 3,527,526 to Silvertooth discloses a catoptric image-forming system wherein the primary mirror twice reflects radiant energy. The secondary mirror reflects the radiant energy once or twice before forming a focus close to the secondary mirror. The various image-forming systems are telephoto lenses that form an image of an object located at or near infinity at a focal plane positioned comparatively close to the secondary mirror. The mirrors disclosed by Silvertooth are preferably aspherical and are not concentric.
A known design for a telescope objective uses concentric spherical mirrors in a four reflection Cassegrainian mirror optical system. The secondary mirror receives light from infinity. The primary mirror focuses the light relatively close to the back side of the secondary mirror. The secondary mirror in this telescope objective must be large relative to the primary mirror. The diameter obscuration of this telescope objective is approximately 0.71. The large obstruction caused by the secondary mirror makes this four reflection telescope objective impractical.
The art has also realized that a Cassegrainian microscope objective can form a four reflection optical path. The image produced by the unintentional four reflection path, however, is quite poor and totally unsuited for work at high magnification. Indeed, the four reflection path has been treated, when recognized, as a problem to be eliminated.